[llvm] [flang-rt] Optimise ShallowCopy and use it in CopyInAssign (PR #140569)

[Slava Zakharin via llvm-commits]

================
@@ -437,6 +437,64 @@ class Descriptor {
 };
 static_assert(sizeof(Descriptor) == sizeof(ISO::CFI_cdesc_t));
 
+// Lightweight iterator-like API to simplify specialising Descriptor indexing
+// in cases where it can improve application performance. On account of the
+// purpose of this API being performance optimisation, it is up to the user to
+// do all the necessary checks to make sure the specialised variants can be used
+// safely and that Advance() is not called more times than the number of
+// elements in the Descriptor allows for.
+// Default RANK=-1 supports aray descriptors of any rank up to maxRank.
+template <int RANK = -1> class DescriptorIterator {
+private:
+  const Descriptor &descriptor;
+  SubscriptValue subscripts[maxRank];
+  std::size_t elementOffset = 0;
+
+public:
+  RT_API_ATTRS DescriptorIterator(const Descriptor &descriptor)
+      : descriptor(descriptor) {
+    descriptor.GetLowerBounds(subscripts);
+    if constexpr (RANK == 1) {
+      elementOffset = descriptor.SubscriptByteOffset(0, subscripts[0]);
+    }
+  };
+
+  template <typename A> RT_API_ATTRS A *Get() {
+    std::size_t offset = 0;
+    // The rank-1 case doesn't require looping at all
+    if constexpr (RANK == 1) {
+      offset = elementOffset;
+      // The compiler might be able to optimise this better if we know the rank
+      // at compile time
+    } else if (RANK != -1) {
+      for (int j{0}; j < RANK; ++j) {
+        offset += descriptor.SubscriptByteOffset(j, subscripts[j]);
+      }
+      // General fallback
+    } else {
+      offset = descriptor.SubscriptsToByteOffset(subscripts);
+    }
+
+    return descriptor.OffsetElement<A>(offset);
+  }
+
+  RT_API_ATTRS void Advance() {
+    if constexpr (RANK == 1) {
+      elementOffset += descriptor.GetDimension(0).ByteStride();
+    } else if (RANK != -1) {
----------------
vzakhari wrote:

```suggestion
    } else if constexpr (RANK != -1) {
```

https://github.com/llvm/llvm-project/pull/140569